Throughout the history of electrical and electronics devices, various apparatus have been designed that require differing characteristics of power supply. Often, the electrical power sources that are available do not have characteristics that such devices require. For example, devices may require either alternating current (AC) or direct current (DC) power. AC devices may be single phase or poly phase (most commonly three phase), or may have various frequencies. Additionally, the operating voltage, minimum or maximum current, frequency, etc. of the power source may vary. Electronic power converters are frequently used in these applications to match the available power supply to the characteristics of a particular device.
In other applications, manipulation of the characteristics of the supplied power is used to control the operation of a device. One such example is the variable frequency drive (VFD) for an induction motor. It is well-known that varying the frequency of the alternating current supplied to an induction motor will vary its rotation speed, which is useful in many industrial applications. It is further known that control of the voltage and current is also required to optimize operation and avoid damage to the motor in such use. Again, electronic power converters are used to take whatever supply of electrical power is available and adapt it as necessary.
Another application of the electronic power converter is the uninterruptible power supply (UPS), in which two or more power sources are connected to a load to prevent interruption of one power source from disrupting operation of the load. Such systems are frequently used with computers and networking equipment as well as other critical loads such as industrial control systems, hospital life support systems, etc. In many cases, the primary power source is an alternating current source while the standby source is a battery. Ideally, in such arrangements, a single power converter can be adapted to provide whatever type of power supply is required for the load from either power source (DC vs. AC, single vs. poly phase, etc.)
For these and other applications, a variety of electronic power converters have been developed. Of particular interest are a certain class of “universal” power converters, which are identified by their ability to accept any of a DC, AC single phase, AC poly phase input of any voltage, frequency, and current characteristics and generate any desired output, whether DC, AC single phase, or AC poly phase, having any desired voltage, frequency, and/or current characteristics. Such universal converters typically comprise three primary components: (1) an input stage, which typically takes the form of a controlled rectifier; (2) a link or storage stage, which has historically taken the form of a DC link including a relatively large storage capacitor; and (3) an output stage, typically in the form of some sort of inverter. The controlled rectifier of the input stage is typically formed from a plurality of semiconductor devices, which could be either thyristors, insulated gate bipolar transistors (IGBTs), or some form of power transistors. The output stage is also typically formed from some form of power transistor or thyristors.
One problem with such topologies has been the DC link, which, as noted above, typically includes a relatively large storage capacitor. These devices can be bulky and expensive, as well as prone to failure. All of which are generally considered to be undesirable. Very recently, electronic power converters based on partially AC resonant circuitry (rather than DC links) have been proposed. One such converter is disclosed in U.S. Pat. Nos. 7,778,045 and 7,599,196 to William Alexander, entitled “Universal Power Conversion Methods,” and “Universal Power Converter,” which are hereby incorporated by reference in their entireties. While Alexander's converter solves the historic difficulties associated with the DC link capacitors of the prior art, his topology relies on bi-directional switches, which unnecessarily increase the parts count, parts cost, and complexity of the device. Additionally, his topology needs a relatively large link inductor which is bulky and expensive. Moreover, Alexander's topology is suitable for the cases that the input and output appear as voltage sources, which is the reason placing the filter capacitors at input and output terminals is mandatory.
In Lipo, T. A.; “Recent Progress In The Development In Solid-State AC Motor Drives,” Power Electronics, IEEE Transactions on, vol. 3, no. 2, pp. 105-117, April 1988 two resonant converters named series resonant AC link converter drive and parallel resonant AC link converter drive are introduced. In Lipo's converters, the link is resonating all the time and the current and voltage of the link are sinusoidal. Additionally, the link inductor and capacitor are both required in Lipo's converters.
A partial-resonant AC-AC Buck Boost converter is described in Kim et al., “New Bilateral Zero Voltage Switching AC/AC Converter Using High Frequency Partial Resonant Link,” Korea Advanced Institute of Science and Technology, (IEEE 1990). In the converter proposed by Kim the link current has a DC component, which significantly reduces the utilization of the inductor/capacitor. Moreover, the resonating time during which no power is transferred is much longer in the converter proposed in Kim et al.
For these reasons, a need exists for a resonance-based, universal power converter having a reduced parts count and improved operating characteristics.